Image multiplier



Jan. 27, G w A IMAGE MULTIPLIER Filed Sept. 21, 1956 INVENTOR.

GEORGE W BAIN 4 0% 4% A TTORNE Y United States Patent '0 llvlAGEMULTIPLIER George W. Bain, Fort Wayne, lnd., assignor to InternationalTelephone and Telegraph Corporation Application September 21, 1956,Serial No. 611,198

6 Claims. (Cl. 250-213) This invention relates to image multipliers ofthe electron type and is particularly directed to means for improvingthe resolution of the amplified image.

Image multipliers of the Weiss type comprise a planar photocathodedisposed parallel to one, two or more finemesh dynodes, and a phosphoranode. The sides of the dynodes facing the cathode are coated with asemi-insulating material capable of high secondary emission. The

potential between the cathode and first dynode is relatively high, sothat an electron image originating at the cathode produces a like imageon the first dynode. The secondary electrons are drawn through theinterstices of the first dynode to the second dynode, where themultiplying action is repeated. Unfortunately, the secondary electronsemitted at the face of the first dynode have random velocities anddirections and some of the electrons will move across the face of thedynode and enter holes removed from the point of origin of theelectrons. This phenomenon, of course, confuses the information amongthe picture elements and reduces picture resolution.

The object of this invention is to provide, in an image multiplier,means for retarding secondary electrons normal to the face of the firstdynode toward the cathode so that all secondary electrons will passthrough openings adjacent their points of origin.

The objects of this invention are attained by spacing a coarse-meshscreen with high transmissivity immediately in front of the first dynodeto produce a retarding field and to turn all secondary electrons towardand through the first dynode.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawings, wherein:

Fig. 1 is a half-sectional view of an image multiplier embodying thisinvention; and

Fig. 2 is an enlarged detail view with field strength lines of theregion immediately surrounding the first dynode of the tube of Fig. 1.

In Fig. 1 is shown an electron multiplier with envelope 1 having twoparallel windows 2 and 3. The windows are of good quality glass withhigh light transmissivity for the visible light region. A photocathode13 is prepared on the inner face of window 2. While composition of thephotocathode is unimportant to the understanding of this invention, itis suggested that the cathode be prepared by caesiating, in the usualmanner, a film of antimony evaporated over a high transparencyconductive electrode on the area to be sensitized. The inner surface ofwindow 3, on the other hand, is prepared with a film of any phosphormaterial 14 which luminesces when bombarded with electrons. Such aphosphor anode, for example, may comprise zinc sulfide (ZnS).

Two dynode electrodes 4 and of extended area are disposed in spacedparallel relation between the windows. Three, four, or more, dynodes maybe employed, if desired. Each dynode electrode preferably embodies acircular frame carrying a taut screen with minute, evenly spaced holesthroughout the area of the screen. The screens may be woven or producedby the photo and acid-etching technique. Conveniently, the screens maycomprise sheets of nickel or copper with holes etched therethrough about300 per linear inch. Good results are obtained when the area of theopenings thus formed comprise 30% to 50% of the area of the screen. Theforward face of each screen, facing the cathode, is prepared for highsecondary emission by evaporating a material of proper semi-conductingproperties on the face of the screen. Such material may be magnesiumoxide, MgO, evaporated over magnesium metal or silver metal to asufiicient depth to insulate the underlying metal conductor frombombarding electrons, Grids 4 and 5 are preferably made with 300 to 500mesh with holes about .002 inch diameter.

According to an important feature of this invention, the screenelectrode 10 is disposed between the cathode and the first dynodeelectrode. The screen 10 may, desired, be constructed similar to thedynode electrodes with a supporting frame and a tautened planar screenin the frame. The openings in screen 10, however, are large compared tothe openings in the dynode electrodes and the metal of the screen isrelatively fine so that the transmissivity of the screen is highcompared to the transmissivity of the dynode electrodes. For example,conductors and openings of the screen 10 may be so selected as tointercept as little as 5% or 10% of the collimated primary electronsfrom the cathode. That is, the transmissivity of the screen 10 may be ashigh as to when, for example, a mesh is woven of .001 inch wire with.020 inch spacings.

The electrodes of the image amplifier of this invention are energizedwith potentials that increase progressively from the cathode to thephosphor anode. Conveniently, the electrodes are tapped to thepotentiometer, as shown. Where the spacing between the cathode andscreen 10 is .050 inch and the spacing between the screen 10 and dynode4 is .020 inch, and the spacing between dynodes 4 and 5 is .005 inch,and where the spacing between the last dynode (5) and the phosphor anode3 is .010 inch, the cathode may be operated at 200 volts, grid 10 atvolts, dynode 4 at zero volts, dynode 5 at +200 volts, and phosphoranode 3 at +4000 volts. Such voltages, which are mentioned merely by wayof example, repress electron emission toward the cathode. The gradients,it has been found, should be at least 400 volts per inch in front of thedynodes. That is,'for a grid to dynode spacing of .020 inch, the voltagedifference between grid 10 and dynode 4 should be at least 80 volts(.020 x 400 volts per inch). As shown in Fig. 2, equipotential lines 12are formed along the front face of the first dynode with distinct dipsinto the openings of the dynode. Hence, secondary electrons having axialvelocity toward the cathode are retarded and turned toward and into theopenings. A negligible number of electrons have sufficient velocity toescape to the next adjacent opening. It follows that picture resolutionis materially improved.

While the principles of the invention have been described in connectionwith specific apparatus, it is to be clearly understood that thisdescription is made only by way of example and not as a limitation tothe scope of the invention.

What is claimed is: 1. An electron discharge device comprising aphotocathode, a plurality of multiplier dynode grids and a '3 phosphoranode, said cathode, grids, and anode being planar and in parallelspaced relation for amplifying electron images originating at saidcathode, characterized in that a planar grid of relatively high electrontransmissivity compared to the transmissivity of said multiplier grids.

is disposed between said cathode and the first multiplier grid andcircuit connections are provided for impressing on said last-named grida potential higher than the potential of said cathode and lower than thepotential of said multiplier grid thereby to decelerate secondaryelectrons from the first multiplier grid.

2. In an electron discharge device: a plurality of parallel spacedplanar electrodes comprising in succession a photocathode, a relativelyopen mesh grid, a plurality of relatively closed mesh grids treated onthe cathode-side to enhance secondary emission, a phosphor anode, andcircuit connections for impressing on said open mesh grid aj potentialhigher than the potential of said photocathode and lower than thepotential of said closed mesh grid adjacent thereto whereby secondaryelectrons emitted from said adjacent closed mesh grid are repelled awayfrom said photocathode.

3. In the combination defined in claim 2, said open mesh grid being of ametal selected for low secondary emission characteristics.

4. An electron discharge comprising a planar photocathode, a parallelspaced phosphor anode, a plurality of planar grid electrodes disposed inparallel spaced relation between said cathode and-anode, the electrontransmissivity of the grid adjacent said cathode being relatively higherthan the electron transmissivity of the remaining grids, said remaininggrids having relatively higher secondary emission characteristics thansaid first grid, and circuit connections for impressing on said adjacentgrid 21 potential higher than the potential of said photocathode andlower than the potential of the grid on the side of said adjacent gridremote from said photocathode whereby secondary electrodes emitted bysaid remote grid are repelled away from said photocathode.

5. In the combination defined in claim 4, means for applyingsuccessively higher potential between the anode, grid, and cathodeelectrodes, the voltage gradient at the front surface of said remotegrid being of the order of 400 volts per inch.

6. In the combination defined in claim 4, means for applying voltages tothe mentioned electrodes to produce at least'400 volts per inchdecelerating field for the secondary electrons in the openings of saidremote grid.

References Cited in the tile of this patent UNITED STATES PATENTS

